Debugging, Disassembly & Reversing in Linux for x64 Architecture/

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Instructions II

Instructions II

Learn about different modes like conditional jumps, register structures, return values, and byte registers.

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Conditional jumps

Consider these two C/C++ code fragments:

if (a == 0)             if (a != 0)
{                       {
    ++a;                    ++a;
}                       }
else                    else
{                       {
    --a;                    --a;
}                       }

CPU fetches instructions sequentially, so we must tell the CPU that we want to skip some instructions if some condition is met (or not met)—for example, if a != 0.

The JNZ (jump if not zero) and JZ (jump if zero) instructions test the ZF flag and change %RIP if the ZF bit is cleared for JZN ...